The demonstrated role of the midbrain reticular formation in the elaboration of behavior makes it imperative that its descending projections be understood. Behavioral and physiological experiments indicate that these projections have a wide but specific distribution pattern in the caudal brain stem and are thereby capable of influencing a large number of somatic and visceral functions. However, there is little anatomical information regarding the descending projections of the midbrain reticular formation. The objective of this project is to study the fiber systems descending from five separate regions of the midbrain reticular formation in the cat. The autoradiographic tracing method will be used exclusively since it is the only neuronatomical method capable of selectively demonstrating fiber systems arising in regions containing fibers of passage. Minute injections of H3-amino acid will be placed in the different regions and the transport of labeled protein will be traced by light autoradiography from neuron cell bodies to their terminals. For each region experiments will be performed utilizing fast axoplasmic transport to more selectively label fields of termination and fast plus slow transport to demonstrate the course of the fiber systems as well as their terminal regions. For each type of experiment the brain stem will be viewed in both the transverse and sagittal planes to gain a more accurate appreciation of the trajectory of the fibers and to ascertain more clearly the loci of termination.